A method for carbonylating methanol with carbon monoxide in the presence of a rhodium catalyst to produce acetic acid is well known as so-called “Monsanto method”. There are two methods for the carbonylation method. One is a method in which acetic acid is used as a solvent, methanol of a raw material is added to the acetic acid, a rhodium compound as a catalyst is dissolved therein and a carbon monoxide gas is fed into the reaction mixture (homogeneous catalytic reaction). The other is a method in which a solid catalyst having a rhodium compound carried on a carrier is suspended in the reaction mixture instead of dissolving the rhodium compound into it (heterogeneous catalytic reaction). However, in both cases, an iodide compound such as methyl iodide is added into the reaction mixture as a cocatalyst (reaction promoter), so that about several tens to several hundreds of ppb (μg/kg) of the iodide compound remains in the acetic acid produced by the carbonylation method even after the acetic acid has been refined by distillation. The iodide compound remaining in the acetic acid in such a manner acts as a catalyst poison to a VAM (vinyl acetate monomer) synthetic catalyst when the acetic acid is used as a raw material of VAM for instance, and accordingly needs to be removed into a level of about several parts per billion.
There is a method for removing an iodide compound remaining in acetic acid by passing the acetic acid through a packed bed of a macroporous-type cation-exchange resin having silver ion or mercury ion exchanged and carried (Japanese Patent Publication No. H05-021031). This method is effective for efficiently removing the iodide compound from the acetic acid and decreasing the iodide concentration of outflowing acetic acid into 10 ppb or lower, but it has a problem that as the carbon number of the iodide compound increases, an adsorption rate decreases, a width of an adsorption zone is widened and a silver utilization at a breakthrough point decreases. As a result, the method can treat a small amount of acetic acid per unit resin volume, which is not favorable from the viewpoint of a treatment cost.
In order to solve the above described problem several methods have been investigated. One is a method in which an ion exchange resin having an active site only on a surface is used, which method is developed through having paid particular attention to the point that the diffusion of an iodide compound in adsorbent particles limits an adsorption rate (Japanese Patent Application Laid-Open No. H09-291058). Another one is a method in which an iodide adsorption apparatus is operated at a temperature higher than about 50° C. (Japanese Patent Application Laid-Open No. 2003-527963). However, the former method has a disadvantage that it is not easy to prepare an ion-exchange resin so as to have the active site only on the surface, and that if the inside of the ion-exchange resin particle is consequently not used effectively, an exchange capacity per unit volume of resin becomes small. On the other hand, the latter method has a disadvantage that when the apparatus is operated at a high temperature, the active site is more rapidly decomposed and released, and silver ion is also more rapidly released.
In addition, a method is proposed which starts an iodide removal operation at a low temperature, and every step of time when an iodide compound is detected in a discharged liquor due to decrease of an iodide removal rate, increases the temperature, so as to reduce the release of an active site and the release of silver ion (Japanese Patent Application Laid-Open No. H09-291059). However, the method also have a disadvantage that it is a complicated operation to increase the temperature step by step, and that the active site unavoidably decomposes and is released and silver ion is unavoidably released, because the exchange resin finally contacts with a high-temperature liquid by any means. The released active site and silver ion become impurities in a product of acetic acid, which is not preferable.